1. Field of the Invention:
The present invention relates to a cryogenic insulating structure for insulating tanks or pipes used for storing or transporting low boiling liquids, such as liquefied methane, liquefied natural gas and liquid nitrogen.
2.Description of the Prior Art
Various methods for insulating tanks containing low boiling liquids are known. From the viewpoint of heat insulating materials, the methods have been classified as solid-gas composite types (fiber, powder, foam) and vacuum types. In the vacuum type special structures for maintaining vacuum are required, so that the expense is high and practical applications are thus limited. In the solid-gas composite type, pearlite power has been used as the fiber-or powder-filled insulating layer.
Heat insulating layers have been used to insulate the outsides above ground LNG storage tanks. However, formation of void spaces caused by sedimentation of the powder has produced difficulties. Accordingly, the addition of supplemental amounts of the insulating powder and the maintenance of the insulation are difficult so that this type of insulation cannot be used practically for shipping tanks.
On the other hand, a foam product composed of a closed cell foam, such as a rigid polyurethane foam, possesses excellent heat insulation capabilities abilities and can be prepared at the site of tank construction. With this type of insulation the maintenance of the heat insulating layer is simpler. Accordingly, such heat insulating material is quite useful and has been used as outer heat insulating material on tanks and pipes for the transportation of liquefied natural gas. However, in connection with the outer heat insulating method, expensive metals which are not brittle at ultra low temperature, such as nickel-steel alloys, or aluminum must be used for construction of the tanks to contain the cryogenic liquid. Accordingly, the cost of tank construction is quite high, and therefore, an inner heat insulating method has been in demand. However, inner heat insulating layers must have the following characteristics:
A. No cracking in the heat insulating layer at cryogenic temperatures.
B. No permeation of the cryogenic-liquid into the heat insulating layer during practical use.
C. A protective layer for preventing leakage of the cryogenic-liquid by accidental breakage of the inner surface of the heat insulating layer.
d. The heat insulating layer should be durable to bending of the vessel caused by the charging or discharging of the cryogenic-liquid and durable to the impact weight caused by movement of the cryogenic-liquid during transportation.
A method of forming a heat insulation layer of rigid polyurethane foam on the inner surface of a cryogenic-liquid tank has been proposed wherein the lining is made by bonding rigid polyurethane foam reinforced with glass fiber or other fibers onto the inner surface of the tank. However, this method requires considerable labor for the preparation of the foam, the bonding operation and the testing of the bonded surface, whereby the cost of such a heat insulating method is prohibitively expensive and is not suitable for large tank construction. A need exists, therefore, for an inexpensive and facile method of insulating the inside of cryogenic liquid tanks.